Central Power Control for Remote Flash Units

The idea of this project is to build the following flash system with the central power output control. The camera uses its built-in flash to trigger the control unit, which in turn triggers the flashes using telephone wires. The control unit also controls the power outputs manually.

The Clip art was downloaded from http://www.clipartheaven.com/

This project can be divided into 3 parts:

1. Optical Slave Trigger

2. Output Power Control

3. Hot shoe, Clamp and Integration

Optical Slave Trigger for Dual Flashes

This slave trigger supports 2 external flash units which can be of different models (e.g. HV and LV). It outputs a pulse to trigger the external flash units once counted the 2nd pulses. It is compatible with digital cameras which use a double flashes.

The circuit is pretty simple and can be built without using PCB.

Parts

I modified a small LED reading light to accommodate the electronics. It saved me some work on building the battery compartment and the mounting bracket of the solar cell.

The solar cell is salvaged from a dual power calculator.

I used phone jack instead of PC jack because it is much cheaper.

Other parts should be readily available.

Principle of Operation

It is basically a 4-bit counter circuit. It outputs a pulse upon receiving the 2nd pulse from the solar cell. It is important that the reset switch is pressed prior to pressing the shutter (a bit user unfriendly) or it won't work.

The solar cell needs to face the built-in flash within a few inches in order not to be miss-triggered by flashes of other cameras at the same scene. It needs to be tightly coupled to the built-in flash if the ambient light is bright.

Output Power Control for Flash Units

Very often, we want to set the output power of the flash unit manually rather than using the A-mode or TTL. Some models like Vivitar 285 have the feature. In this project, we will modify flash units so that we can manually vary the output power similar to the Vivitar 285.

Warning

This project requires experience of opening and re-assembling YOUR flash unit(s). It also requires appropriate safety measure against the high voltage parts that becomes exposed when the flash unit is opened. The high voltage will remain in the circuitry even the power is shut down, and can cause injury, death or other damage if touched. Inexperienced person should do it under supervision of an experienced person.

Disclaimer

The author specifically disclaims any and all liabilities for any of your damages arising from following this tutorial. The information provided here is for sharing the experience only.

Choice of flash models

1. It is required that the flash unit has A-mode which is independent of camera's metering system. Some models have both A-mode and TTL, and have more than one pin at the hot shoe. They can still be used but only A-mode will be required.

2. Flashes with TTL and/or M-mode only are not suitable for this project.

3. Select the GN you need.

4. Select those models that you can open and re-assemble. (I have a Metz 32CT4 and , so far, I don't know how to open it ! )

Modification

1. Open the flash unit and discharge the biggest capacitor with a 100 ohm ~ 500 ohm, 5W resistor as following:

2. Identify the photo diode. It is a 2 pin semiconductor which has an optical cap facing the external of the flash unit. Replace the photo diode with a 50K potentiometer (with a 1 K resistor in series with it).

3. Very unlikely that there is room for installing the potentiometer inside the flash unit. I installed a small 2-pin connector for the external connection. That way, both wired remote control and local control can be done readily. Re-assemble the flash unit. That is it!

Integration

1. DIY Hot Shoe

As hot shoes are not readily available here in Vancouver, I modified a broom holder to hot shoes.

The clamps need some work.

1. Make the slot to shape.

2. File the edges so that they can make electrical contact with the flash unit.

3. Drill holes and add a screw and a nut.

4. Try fit with the flash unit.

5. Cut a piece of PCB with width equal to the inner width of the clamp. Paste a copper strip in the middle for electrical contact with the flash unit. A push button is added for testing the contacts. A 2-pin connector is installed for means of connection to remote power output control. See the schematic diagram below.